Strain-tunable electronic properties and lithium storage of 2D transition metal carbide (MXene) Ti₂CO₂ as a flexible electrode

Two-dimensional MXenes have attracted considerable interest as promising flexible electrode candidates due to their favorable flexibility and high specific capacitance. Thus, taking Ti₂CO₂ as the representative MXene, we have investigated the strain effects on electronic properties with focus on lithium adsorption and structural transformation by using first-principles calculations. It is found that the indirect band gap in pristine Ti₂CO₂ changes to a direct band gap with tensile strain and closes with compressive strain. The adsorption of the first Li layer will attract the unoccupied nearly free electron (NFE) states in pristine Ti₂CO₂ down to the Fermi level in Ti₂CO₂Li₂ while the adsorption of the second Li layer results in little change in the electronic structures owing to the pinning effects of NFE states near the Fermi level in Ti₂CO₂Li₂. Moreover, the surface adsorption of Li atoms up to bilayers on Ti₂CO₂ is predicted to be energetically favorable and could be further stabilized by suppression. Meanwhile, the theoretical exploration predicts a possible decomposition of Ti₂CO₂Li₄ back to Ti₂C and Li₂O under critical tensile strains. This work provides encouraging results for the strain-tunable conduction behavior and lithium storage in Ti₂CO₂ and further sheds light on the design and selection of electrode candidates for flexible energy storage applications.